The present invention relates to a bobbin basket for a sewing machine, and more particularly, to a horizontal lock-stitch bobbin basket.
Known bobbin baskets for sewing machines are made of steel or the like. Such baskets, however, have disadvantages such as:
Generally, there is provided a peripheral bearing rib on the outer surface of the cylindrical side wall of the bobbin basket. For journalling the bobbin basket within the raceway of a rotary hook assembly, the cooperation of the raceway and the rib needs to be oiled to secure a smooth rotation therebetween. Unfortunately, it is unavoidable that the thread, and thus in turn the cloth to be sewn, will be contaminated by the lubricating oil between the raceway and the bearing rib.
Owing to needle fragments that fall between the rotary hook and the bearing rib of the bobbin basket and to the resulting wear occurring therebetween, or to occasional vibration, leading parts of the bearing rib and the raceway will become chipped. When the chipped areas grow to any substantial extent, non-alignment between the bobbin basket and the rotary hook will cause thread breaking.
Furthermore, an all steel hook assembly by comparison wears out sooner than an assembly using a plastic bobbin basket. The life of a steel bobbin basket is about three times shorter than a plastic one because of the heat generated in the steel assembly.
Moreover, the steel costs are higher than the plastic. The cost of manufacturing a plastic bobbin basket is about one-fourth of that of a steel one.
In addition, the bearing rib of the steel bobbin basket and the raceway of the rotary hook will exhibit greater wear than an assembly using a plastic bobbin basket will exhibit, since the steel bobbin basket is heavier and the coefficient of friction between the rib of the steel bobbin basket and the steel raceway of the rotary hook is larger than that for an assembly using a plastic bobbin basket. The worn rib and raceway will cause unbalance in the assembly, thereby interfering with correct release of the needle thread.
The applicant has sought to overcome the above encountered by the prior art.